Axon degeneration (AxD) is a hallmark feature of a variety of neurodegenerative diseases, such as Alzheimer's disease and Parkinson's disease, as well as various neuronal injuries, such as traumatic and ischemic brain injuries, and peripheral neuropathies. A major component of AxD is the presence of an endogenous degeneration program that, when activated, results in axonal self-destruction. The widespread occurrence of AxD in both disease and injury states suggests that therapeutic interventions that protect axons from degeneration could have a profound impact on human health and quality of life. In spite of the clinical importance of AxD, there is a critical gap in knowledge regarding th mechanisms by which the degeneration program is activated and how activation of program components relates to the progression of AxD. The recent identification of Sarm1 as an essential component of this program suggests that furthering our understanding of Sarm1 may have profound implications for our broader understanding of AxD mechanisms and progression. I have developed a Sarm1 biosensor that allows visualization of injury-induced Sarm1 activation in axons using ratiometric FRET microscopy. In Aim 1 this biosensor will be optimized and used to map injury-induced Sarm1 activation in axons and correlate it with important AxD markers by using Ca2+-imaging, mitochondrial dyes, and morphological analysis to determine how the onset and progression of AxD features relates to Sarm1 activation. The biosensor will also be used to test if the preservation of the proximal axonal segment after injury is due to differential Sarm1 activation. Aim 2 will use the Sarm1 biosensor to identify mechanisms regulating injury-induced Sarm1 activation. Intrinsic regulatory features of Sarm1 will be identified by mutating residues in the biosensor that are predicted to undergo posttranslational modification (PTM) and testing the effect on biosensor activation and AxD in both injured and uninjured axons. Enzymes predicted to mediate PTMs identified as being important will be tested using shRNAs and pharmacological inhibitors. In parallel, known modulators of AxD will be tested to determine if they act by modulating Sarm1 activation. Lastly, an enriched pool of shRNA targets previously identified by our lab as protecting against AxD will be screened using the biosensor to identify novel upstream molecular components mediating injury-induced Sarm1 activation. These aims will fill a critical gap in knowledge regarding the mechanisms of AxD program activation and its relationship to AxD progression. By 1) defining the relationship between Sarm1 activation and AxD progression and 2) identifying the mechanism(s) of injury-induced Sarm1 activation, this proposal will significantly enhance our fundamental understanding of AxD and may identify novel molecular targets for therapeutic intervention in the treatment of neurological disease and injury.